With the growth of the wireless communications industry, wireless communications protocols become more sophisticated and demanding in their requirements for complex modulation schemes and narrow channel bandwidths. The ultimate goal is to encode as much digital information as possible in a given channel. One such modulation scheme for encoding digital information is polar modulation. Polar modulated RF transmitters utilize both amplitude modulation and phase modulation to maximize the amount of information that can be encoded with minimum bandwidth. By using multiple combinations of phase and amplitude, multiple digital bits of information can be represented. Large signal amplitude modulation allows several distinct levels of modulation with adequate noise margins for reliable encoding of digital data. However, in a polar modulated system, large signal amplitude modulation can interfere with proper operation of phase modulated (PM) signals. The bandwidth of transmitted polar modulated RF signals must be contained within a single channel. Output Radio Frequency Spectrum (ORFS) is a measure of adjacent channel interference, which must be minimized. Some polar modulated RF transmitters may use pre-distortion methods with polynomial curve fitting to meet RF spectrum requirements.
In a typical polar modulated RF transmitter, PM signals follow a conventional signal path. An RF modulator receives a phase modulation signal and phase modulates an RF carrier to produce a PM RF signal, which may then be amplified by one or more RF driver amplifier stages that feed an RF final amplifier stage as part of an RF power amplifier. The RF final amplifier stage amplitude modulates the PM RF signal to create a polar modulated RF output signal. The RF final amplifier stage receives an envelope supply voltage from an amplitude modulated (AM) power supply, which provides the amplitude modulation in the final stage. In some designs, an RF final amplifier stage may use one or more bipolar NPN transistor as an active element, in which the collector of the NPN transistor is coupled to the envelope supply voltage. The PM RF signal is fed to the base of the NPN transistor. In the presence of large signal amplitude modulation signals, the voltage at the collector of the NPN transistor may drop below the voltage at the base of the NPN transistor, resulting in forward biasing of the base-to-collector junction, which distorts the amplified PM RF signal and degrades the RF spectrum. Therefore, to meet ORFS requirements and minimize pre-distortion requirements, there is a need for a polar modulated power amplifier that does not forward bias its base-to-collector junction in the presence of large AM signals.